handbook: Add a note about loading vmm.ko on boot

[emaste]

Summary

Event:          Kitchener-Waterloo Hackathon 202506
Sponsored by:   The FreeBSD Foundation

(Report from https://chaos.social/@txt_file/114711667877702884)

[emaste]

This could instead or in addition be a reference to the Persistent Configuration section later in the document -- but either way I think it's good to provide a reference here.

[0mp]

I'd change that to:

The Permanent Configuration section explains how to load the kernel module on boot.

or

The Permanent Configuration section explains how to make the configuration persistent across reboots.

The second example could be added below the networking code snippet.

[carlavilla]

Can you please use sysrc?
Fixable on commit